People are still wondering what made former U.S. presidential candidate Dr. Ben Carson endorse his former rival Donald Trump on Friday considering how the latter had said unpleasant things about him during the campaign.

Trump once called Carson a "pathological liar" and even compared his temper to that of a child molester, according to Business Insider.

In return for this, Carson praised Trump for his "guts" and "energy" when he joined the billionaire businessman at his resort in Palm Beach, Florida on Friday, NBC News reported.

In an opinion piece for Business Insider, Josh Barro said Carson "had perfectly logical self-interested reasons for endorsing Trump"—just like what Gov. Chris Christie of New Jersey, another former Trump presidential rival, earlier did.

"Politicians are always tempted to back the inevitable winner, in an effort to get in his or her good graces. But a Trump endorsement is more likely than usual to pay off in the form of power or influence within an administration," Barro said.

When he appeared with Trump at the latter's news conference on Friday, Carson was asked whether he would be interested in serving in Trump's administration. Carson replied that he and Trump had already agreed he would be involved in formulating policy.

"I would be very interested in helping to achieve the goal of saving America and making it great," he said.

Carson portrayed a rosy image of Trump far different from that of an angry demagogue that his critics have labelled him.

"There are two different Donald Trumps," Carson said. "There's the one you see on the stage and there's the one who is very cerebral, sits there and considers things very carefully. You can have a very good conversation with him. That's the Donald Trump that you're going to start seeing more and more of."

Asked about Trump's past criticisms of his candidacy, Carson said, "We buried the hatchet. That was political stuff."

Carson said he opposed efforts within the Republican Party to derail Trump's candidacy.

"What I have seen recently is political operatives ... trying to thwart the will of the people," Carson said. "I want the political process to play out."

For his part, Trump said winning the endorsement of a former rival is part of his effort to unite the Republican Party, USA Today reported.

"It adds total credence to what I'm trying to do," he said in introducing Carson during a news conference.

The Carson endorsement came the morning after a more subdued Republican presidential debate, which Trump said was another indication that the party is uniting for good.

"It was fun, it was different," Trump said of the debate.